Designing a Solar Heated Structure (Part One)

On December 06, 2007 in Solar Cabin

Apologies for the stagnation of the Solar Cabin construction. While I lay in waiting for materials and capital, I have been doing lots of design work. As I was running through calculations and doing research, I realized how daunting this might be for many people looking to approach this from a “Do-It-Yourself” standpoint.

And from that random though comes “Part I” of my step-by-step guide to designing a solar heated structure:

“Designing a Solar Heated Structure“

When heating a structure using a “direct gain passive solar” system (direct gain means the space is directly heated by the sun) There are many different aspects that need to be considered. Initially, it is good to come up with a plan consisting of a rough sketch of your ideas, and any important details you think of as well. As the plan progresses, and you do more calculations and design, the plan can be developed into something more detailed and defining of the structure to be built.

In the beginning, you will need to consider the basics of Direct-Gain Solar Design. It is important that the structure be placed such that it gets maximum sun exposure to the collectors (in this case, the windows), The collectors need to be appropriately sized for the space, and there needs to be sufficient thermal mass to store heat, as well.

Of course, heat loss cannot be discounted. Attention needs to be paid to insulation, as well as infiltration and the heat loss through walls, windows, doors, ceilings, floors etc. Here is an excellent article on heat loss in buildings from engineeringtoolbox.com that will be used later on in the “guide.”

For now, though, we will concentrate on site selection and interior layout; two very important steps in designing a solar heated cabin, home, or commercial/industrial building. If the building’s location and orientation are not factored into the construction, the so-called solar heated structure will likely be a disaster. “Measure twice, cut once” and all that..

In the winter, the sun is low in the sky during the evening hours, causing the northern side of a building have little in the way of “solar gain.” Because of this, a solar heated building will benefit from having most of the “heat collectors” on the southern exposure of the building, and the smallest possible size northern exposure. Most decide to elongate the building on the east-west axis, and have a shorter northern wall. Many people now however are experimenting with different shapes and sizes of buildings, so don’t be afraid to be creative (providing the design is sound.)

With a smaller northern side (whether shorter, narrower, whatever) the building will have less exposed surface area on the northern (non sun-gaining) side. This translates into lower heat loss. Combine this with a minimum of doors and windows on the north side, and lots of insulation, and your solar home is starting to be a twinkle in your eye. Don’t get ahead of yourself though, there is a lot more work to be done still even just in the design phase.

Ideally, you are going to want to find a spot for your building that has the minimum number of objects possible obstructing the sun’s rays. And by the minimum, I mean none. There should be no obstructions between 9am and 3pm solar time, in the winter. This is important. I have even changed the planned “exact location” of the cabin a few times just to avoid having to cut down too many/any trees.

You will want to go to the site with a “sun chart” which will help you to determine the position of the sun in the sky at any point in the day. The sun chart will help you to determine if there is going to be anything in the way (accurately, rather than just guessing) of your building’s south side.

On the interior of the building, you will want to pay attention to the orientation as well. Areas that are occupied more often should be on the south side of the building, and areas needing less heat like laundry rooms, closets, storage rooms should be on the northern side. These are often called “buffer spaces” as they help to shield the southern sun-heated living area from heat loss.

After making sure the area is free of obstructions, and laying out the interior of the building, you will want to lay out the building using stakes and string or surveyor’s tape, in its actual location, so that you can get an idea of how the building will look in the surroundings, and verify everything with dimensions.

With respect to entry doors, always give yourself a “mudroom” or “airlock.” By this I mean one exterior door that lets the person entering into a small room where another door needs to be opened to enter the building. This prevents large amounts of heat loss from the interior of the building by having the outermost door closed before opening the innermost door. If this were not implemented, cold air from the outdoors would be exchanged with warm inside air every time the door was opened.

As you may notice, many elements of Solar Heating Design are things that could be, and in many cases are, implemented on other homes as well. Preventing heat loss in the winter time for example, is an excellent way to reduce energy loss. Many people add weatherstripping and insulate doors and windows during the winter in existing homes, as it is, and many homes already have dual entry doors and windows on the south side.

Unfortunately many homes that are built already seem to have been built by someone who had no regard for the power of the sun whatsoever, and very well may be a lost cause if your intention is a mostly solar heated building. The best bet is to start fresh (although that is a hard fact to swallow with the price of building materials.. Try to recycle things if possible.. Tear down an old garage or barn for example, and use the lumber in the construction of your new structure.)

When starting from scratch, you will need to learn about many different aspects of construction, and in most people’s cases, deal with a contractor or several. Foundations need to be laid, frames must be built, and most importantly, codes must be followed. If you are going to be doing this yourself please get a few books on the subject, as well as copies of the building and electrical code, and make sure you study up before going to work.

That is all for now, but make sure to come back for my ramblings on window sizing, insulation, roof design, foundation types, thermal mass, and many other topics. The next step in designing our solar heated structure is deciding how the building will be constructed. By this I mean the foundation type, frame, wall, and roof type all need to be determined. I will go over some of the advantages and disadvantages of many of these construction styles with regards to solar heating, in the next section of this guide.

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